The present invention relates to gas detectors generally, and, more particularly, the invention is directed to an apparatus and method for the detection and measurement of gases in low concentration.
A large and growing market exists for analytical devices which can be used to analyze reaction products of a wide variety of industrial processes. For applications such as the detection of toxic gases, it is necessary to measure the concentration of the species of interest in very low concentrations, typically as low as parts per million. A conventional approach for measuring the concentrations of a gas of interest is to put a sample of ambient air in an optical cell and measure the magnitude of absorption of specific molecules of interest at a specific wavelength in the infrared. Since the concentration of the molecules is expected to be quite low, the infrared absorption to be measured is also very low. In order to increase the magnitude of the absorption, a long path optical cell is typically employed in which, by the use of multiple reflections between mirrors, effective path lengths as long as 20 meters may be achieved. The disadvantage of this approach is that the multiple pass optical cell is expensive, large, and heavy. It does not therefore lend itself to the portability necessary for a device of the type desired for compliance with O.S.H.A. regulations covering permissible levels of these gases in the workplace.
Only one such cell for gas measurement is employed in commercially available instruments so that the cell must be first filled with filtered air to establish a "reference" absorption, then emptied and refilled with ambient air. The absorption is measured a second time and the two absorption measurements are compared to determine the absorption of the gaseous species of interest. This procedure is obviously very time consuming and very susceptible to errors due to drift in the electronics in between measurements. If the concentration of the gaseous species to be measured is very low, the theoretical difference between the two measurements is very small. Two large transmission values must therefore be subtracted in order to yield a small difference signal, and any drift in the large transmission values will mask the true absorption value.
Another weakness in the approach used in presently available instruments is that interference filter wheels are employed to tune a source of infrared light to the appropriate wavelength for the absorption measurement. While the interference filter wheel can achieve the high energy throughput of infrared desired for this type of measurement, it achieves a very low resolution of specific wavelengths in the infrared. This result in interfering absorptions between various gaseous species and again results in errors in measurement.
It is therefore an object of this invention to provide an improved apparatus for the detection and measurement of a concentration of a gaseous species in very low concentrations.
It is another object of this invention to provide a process for the detection and measurement of gaseous species.
It is yet another object of this invention to provide a compact, portable gas detection and measurement device with the capabilities to comply with O.S.H.A. regulation measurements of toxic species.